Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

I need to replace my 18 yr old heat pump. Since it's on the roof of the building I haven't seen it, but I've been told it's 1 1/2 ton. Would 1 ton be enough? This one frequently cuts on and off and from what I've been reading on this board I think that's a sign that it's oversized. What manufacturers carry a 1 ton?

susan talk to the profesional that you plan on doing the work get 3 est for the job ask the same questions of them all I have found that when you here the same answer from differant estamater it is usually the truth dont be supriesed if a 1 ton is more expensive than a 11/2 because they are less in demand also ask the estamater to do a load calc for you. Good luck

I should have given more information---my condo is on the ground floor of a two-story building. Only two walls are exterior, and I only get direct sun in the afternoon. I have no intention of buying any load calc software since most of the questions won't apply to me. I only really need heating or a/c for a total of 2-3 months a year. The rest of the time the heat pump isn't needed. I've been using a space heater this winter and it's been enough for my small condo (temps haven't been below 40 in my area). I'd just get a room air conditioner if I could, but the condo assoc. regs. don't allow that. It's a heat pump or nothing. Or maybe I should just get a ceiling fan.....

Sizing methods

Susan, I am a homeowner who has also wondered whether an AC's on-time vs. off-time (duty cycle) can be useful in knowing whether equipment is sized right. While I am convinced that can be useful, it may be more practical to use "MANUAL J" which is far more familiar to the best experts.

Lots of hacks will try to size according to square footage but that goes against everything Manual J stands for. It may be some work for you to find one or more contractors who can do Manual J calculations, but I would go to the effort if you have any worry about being right sized. Expect to pay most contractors too, for doing Manual J for you -- it's a bit of work and not fair to ask them to work for free. But the winning bidder may well apply that fee toward his installation work.

That said, there are a couple of advantages to measuring duty cycle. I believe it gives valuable information, but only when measured on the peak period of the season, when the duty cycle should come close to 100%. Manual J is something you can do every day of the year, and it will give the same answer every time.

For my own house, I do plan to measure duty cycle this summer using an electronic data logger (similar to "Hobo" but a different brand). This project is easier for me because my climate has about 100 drearily repetitive hot days each summer -- your climate may not. And I have already done a whole-house Manual J calc, intend to do a room-by-room Manual J before installing anything, so the duty cycle method will only serve to increase my confidence that downsizing is appropriate. This decision is important enough that I don't mind going through several methods to be more sure it is correct.

You probably know correctly that you have an over-sized heat pump based on your observing the runtime. But the HVAC contractor who sizes your next system will be more professional if he uses Manual J methodology to confirm that.

Best of luck -- P.Student

P.S. Just read your pooh-poohing of software because "most questions won't apply". You are wrong there, the fact of having only 2 outside walls makes Manual J more subtle but does *not* make it inappropriate. I still think the best decision is to get an experienced person to do Manual J for you, taking those 2 walls (and the limited sun) into account.